Process for preparing silver catalysts

ABSTRACT

A supported silver catalyst for the oxidation of ethylene to ethylene oxide with molecular oxygen is made by impregnating a support with a hydrocarbon solution of a silver salt of a neo-acid having 7 or more carbon atoms, substantially free of water and said neo-acid. Preferably, the hydrocarbon solvent is toluene, xylene, ethyl benzene, cyclohexane, or cumene. The impregnated support is dried and the silver is activated under conditions chosen to provide the optimum selectivity to ethylene oxide. Thereafter, the activated silver catalyst may be impregnated with an alkali metal solution sufficient to increase selectivity of the silver catalyst above its alkali-free state. The amount of alkali metal on the finished catalyst is the optimum for the particular support selected and preferably will be about 1-6×10 -3  gew/kg of catalyst. The catalyst may also include additional promoters, such as the alkaline earth metals, preferably barium.

PRIOR ART

The invention relates generally to the vapor-phase oxidation of ethylenewith molecular oxygen to form ethylene oxide, a reaction of importancein the chemical industry. The reaction is usually carried out over asupported silver catalyst. The invention relates in particular to animproved method of preparing such catalysts.

Many methods of catalyst preparation have been disclosed. In their 1949article in the Canadian Journal of Research, Vol. 27, Sec. B, No. 11, p.815-6, McKim and Cambron list seven methods already available in theart. Many others have been disclosed since that time.

Silver should be dissolved in order for a uniform distribution of fineparticles to be formed as the silver compound is decomposed duringsubsequent activation. This may be accomplished by using silver nitrateas was the case in early preparations of silver catalysts, since silvernitrate is highly soluble in water. Later, silver salts of relativelylow molecular weight carboxylic acids were used. Generally, they areonly moderately soluble in water and use of solubilizingcomplexingagents such as ammonia and amines have been proposed, for example inJapanese Published Application No. 46-19606, U.S. Pat. No. 3,702,259,and G.B. No. 1,533,813. A silver salt often mentioned is silver lactate,which contains a hydroxyl group, accounting for its relatively highsolubility in water, compared to other silver salts of carboxylic acids.Since it is important to prepare a very concentrated silver solution forimpregnating a catalyst support, it will be apparent that silver saltsof higher molecular weight carboxylic acids would not suggest themselvesto one skilled in the art unless provision is made to solubilize thesilver salts in aqueous solutions. The salts of higher molecular weightfatty acids such as silver stearate have been used as thickeners inpreparing hydrocarbon greases, as shown in U.S. Pat. No. 2,391,842. Suchsilver salts have only very limited solubility in water. Despite thisfact, silver salts of higher molecular weight carboxylic acids have beensuggested as potential sources of silver for ethylene oxide catalysts.

Silver salts of fatty acids have been employed as antimicrobial agents.In U.S. Pat. No. 3,255,222 silver salts of various fatty acids,including capric (decanoic) acid are formed and then placed in solutionby using amines and oxygen-containing organic solvents, such asalcohols. The concentration of such salts are very low and thereforewould not be useful for preparing silver catalysts, which may contain upto 15% silver or more. Aqueous ammonia solutions have also been prepared(see U.S. Pat. No. 3,255,223).

Such compounds are related to the metal soaps used as driers and thelike and, as shown in U.S. Pat. No. 2,807,553 they are generallywater-insoluble solids, so that for many uses organic solvents areemployed. Paint driers were prepared from the cobalt, manganese, andlead salts of C₈ to C₂₀ synthetic acids having tri-alkyl acetic acidconfigurations, as described in U.S. Pat. No. 2,955,949.

Silver neodecanoate was preferred as an ingredient in a palladium-silvermixture used to decorate ceramics in U.S. Pat. No. 4,262,040.Preparation of the silver compound was not described.

Despite the frequent reference to the use of carboxylic acid silversalts for preparing silver catalysts, use of the higher molecular weightsalts is not believed to be widely practiced. More recent patentdisclosures suggest that lower molecular weight salts which can bedissolved in water, such as lactic acid, or solubilized by amines suchas silver nitrate or oxalate, appear to be the norm. However, it has nowbeen found that the silver salts of higher molecular weight carboxylicacids, which are quite insoluble in water, can be employed to formactive and selective catalysts by the method of preparation to bedisclosed below.

SUMMARY OF THE INVENTION

A supported silver catalyst for the oxidation of ethylene to ethyleneoxide is made by impregnating a support comprising alumina, silica,silica-alumina, or combinations thereof and having a surface area ofabout 0.2-1.5 m² /gm with a hydrocarbon solution of a silver salt of aneo-acid (as hereinafter defined), the solution being substantially freeof water and the corresponding neo-acid. The impregnated support then isdried and activated by heating to temperatures of about 200° to 600° C.,preferably 250°-500° C., for a sufficient time to produce an activefresh catalyst having an average silver particle size of about 0.1-2microns.

A neo-acid is defined as one in which the carboxylic acid moiety isattached to a carbon atom which is directly attached to three othercarbon atoms, or to other carbon atoms which are so attached. Thepreferred neo-acid is neodecanoic acid, but neo-acids having seven ormore carbon atoms are useable.

Preferably, at least one alkali metal selected from the group consistingof Cs, K, and Rb is added to give the catalyst improved selectivity toethylene oxide. The catalyst may also contain other promoters, such asthe alkaline earth metals, preferably barium. These promoters also maybe salts of neo-acids.

The silver salts of neo-acids may be prepared by various methods and inparticular by reacting a silver compound with a neo-acid in the presenceof a solubilizing agent, such as ethanol. The silver salt may beprecipitated from solution, washed free of residual neo-acid anddissolved in a hydrocarbon solvent, preferably toluene, xylene, ethylbenzene, cyclohexane, or cumene.

The finished catalyst may contain up to about 15 wt % silver and about8×10⁻³ gew/kg (gew=gram equivalent weight) of the alkali metal(s),preferably 5-13 wt % silver and up to about 7×10⁻³ gew/kg alkalimetal(s), most preferably 8-11 wt % silver and about 1-6×10⁻³ gew/kgalkali metal(s).

The support preferably is an alumina containing up to about 15 wt %silica and having a surface area up to about 2 m² /gm, particularlyabout 0.2-1.5 m² /gm, and especially about 0.3-1.0 m² /gm. The preferredsupports will be capable of selectively adsorbing alkali metals fromsolution, and the preparation method of the invention is intended totake advantage of that property.

The amount of alkali metal(s) added will be selected to optimizecatalyst performance and will be dependent upon the surface area of thesupport chosen. That is, more alkali metal will be used on supportswhich have larger surface area than on those having relatively smallsurface area.

The catalyst of the invention may be employed at oxidizing conditionstypical of the art to prepare ethylene oxide by the vapor-phaseoxidation of ethylene with improved results.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Catalyst Composition andPreparation

Preferred Catalysts prepared in accordance with this invention containup to about 15% by weight of silver, expressed as metal, deposited uponthe surface and throughout the pores of a porous refractory support.Silver contents higher than 20% by weight of total catalyst areeffective, but result in catalysts which are unnecessarily expensive.Silver contents, expressed as metal, of about 5-13% based on weight oftotal catalyst are preferred, while silver contents of 8-11% areespecially preferred.

Catalysts may be made with supports comprising alumina, silica,silica-alumina or combinations thereto. Preferred supports are thosecontaining principally alpha-alumina, particularly those containing upto about 15 wt % silica. Especially preferred supports have a porosityof about 0.1-1.0 cc/gm and preferably about 0.3-0.8 cc/gm. Preferredsupports also have a relatively low surface area, that is about 0.2-1.5m² /gm, especially 0.3-1.0 m² /gm. Such surface areas are determined bythe BET method [J. Am. Chem. Soc. 60, 309-16 (1938)]. Porosities aredetermined by the mercury porosimeter method; see Drake and Ritter,"Ind. Eng. Chem. Anal. Ed.," 17, 787 (1945). Pore diameters and porediameter distributions are determined from the surface area measurementsand the apparent porosity measurements.

The preferred support will be capable of selectively adsorbing alkalimetals, particularly potassium, rubidium, and cesium from solutions ofthose metals. By this is meant the deposition of greater amounts ofalkali metals than would be predicted by calculation from the amount andconcentration of the solution absorbed by the support. The mechanism bywhich this is accomplished is not clear, but may involve ion-exchangewith other metal ions found on the support. In this regard, it is ofinterest to note that published British patent application GB No.2,043,481A teaches against the use of supports which contain ionsexchangeable with the alkali metals (page 12, line 50). However, it hasbeen found that the promotional effect of the alkali metals is enhancedwhen the support can selectively adsorb alkali metal ions. The presentmethod of silver catalyst preparation includes steps which are intendedto take advantage of this property of the preferred supports.

For use in commercial ethylene oxide production applications, thesupports are desirably formed into regularly shaped pellets, spheres,rings, etc. Desirably, the support particles used have "equivalentdiameters" in the range from 3-10 mm. and preferably in the range of 4-8mm, which are usually compatible with the internal diameter of the tubesin which the catalyst is placed. "Equivalent diameter" is the diameterof a sphere having the same external surface (i.e. neglecting surfacewithin the pores of the particle) to volume ratio as the supportparticles being employed.

The silver is added to the support by immersion of the support into asolution containing a silver salt of a neo-acid having seven or morecarbon atoms and being substantially free of water and said neo-acid.The silver-containing liquid penetrates by absorption and/or capillaryaction into the pores of the support. A single immersion or a series ofimmersions, with or without intermediate drying, may be used, dependingin part upon the concentration of the silver salt in the solution. Toobtain catalysts having silver contents within the preferred range,suitable impregnating solutions will generally contain from 5-50 wt % ofsilver, expressed as metal, but supplied as silver salts of neo-acids.The exact concentrations employed, of course, will depend upon, amongother factors, the desired silver content, upon the nature of thesupport, the viscosity of the liquid, and solubility of the neo-acidsilver salt.

The impregnating medium, as already indicated, is characterized as asubstantially water-free and neo-acid-free organic solution of a silversalt of a neo-acid having seven or more carbon atoms. As will be seen, ahydrocarbon solvent is employed, such as toluene, cyclohexane, xylene,ethyl benzene or cumene, which would ordinarily be water-free. Sincewater is considered detrimental to the preparation of silver catalystswhen the method of the invention is used, it should be present in nomore than about 0.1 vol. percent in the silver impregnating solution,preferably less than about 0.01 vol. percent.

While higher molecular weight organic acids have been mentioned in thepast as usable for preparing silver catalysts, it will be clear from theexamples given in the prior art that much lower molecular weight acids,such as lactic acid (hydroxy propionic acid) or oxalic acid arepreferred. The silver salts of these acids are somewhat soluble inwater, or can be made so by complexing with ammonia, amines, and thelike. Silver nitrate, also used at times, is soluble in water as well.As the molecular weight of the acid increases, it generally becomes moreorganic in character and is no longer soluble to any appreciable extentin water. Such higher molecular weight acids can also form silver salts,but often they are not soluble in organic liquids. It has been foundthat organic acids having seven or more carbon atoms are useful inpreparing silver catalysts. The tri-alkyl acetic acids, as discussed byKirshenbaum in U.S. Pat. No. 2,955,949 are particularly effective, aswill be seen in the examples. Such acids are available commercially as"neo-acids" and are formed from olefins by carbonylation and hydrolysisor by hydroformylation and oxidation. Since they are soluble in organicliquids their metal salts are used as paint driers and the like, wherethey are more stable than the naphthenic acid salts. The term "neo-acid"for purposes of this invention means that the carboxylic acid moiety isattached to a carbon atom, which is directly attached to three othercarbon atoms, or to other carbon atoms, which are so attached. At leastone of those carbon atoms will be part of a relatively long alkylgroups, which gives the compound its organic character. Neodecanoic acidis particularly preferred. As commercially available it is a mixture of67% 2 ethyl 2 methyl heptanoic acid, 31% 2, 2 dimethyl octanoic acid,and 2% 2, 2 diethyl hexanoic acid. Other commercially available acidsare neopentanoic acid and neoheptanoic acid. Generally, neo-acids having7 carbon atoms or more are usable, although neodecanoic acid is quitesoluble and is preferred. The neo-acid configuration is considerednecessary since those acids having only straight chains of carbon atomsappear to be so insoluble as to make preparation of high silverconcentrations practically impossible.

As indicated, the silver is deposited upon the support by immersion ofthe support into a water-free hydrocarbon solvent containing a silversalt of a neo-acid until the solution has been absorbed into the poresof the support. Typical immersion times of from 1 to 60 minutes attemperatures of from 30° to 120° C. will usually suffice to achievesilver contents of as high as 7-15 wt %, as silver.

In addition to the silver salts, the liquid in which the support isimmersed may contain promoters, such as alkaline earth metal promoterse.g. barium. Contrary to the usual silver solutions which contain water,it is characteristic of the present method to employ substantiallywater-free impregnating solutions, which will not readily dissolve theusual metal salts. However, it is feasible to prepare promoter metalsalts of neo-acids which will be soluble in the solvent selected for usewith the silver salts and thereby to introduce promoter metals into thesilver impregnating solutions. Alternatively, the promoters may be addedby post-deposition after the silver has been deposited, which permitssome latitude in the solutions used, that is, once the silver has beendeposited, it would be possible to use aqueous solutions to depositwater - soluble promoter metal salts.

Contrary to previous practice, it has been found that free acid shouldbe avoided if the best catalyst performance is to be obtained.Previously, it was typical to include some free acid, e.g. lactic acid,along with the silver salt. In the present method, any free neo-acid,that is, unreacted with silver, will be separated, leaving substantiallyonly the silver salt of the neo acid in the impregnating solution.

The catalysts are prepared by impregnating a support with a water-freehydrocarbon solution of a silver salt of the selected neo-acid, followedby activation of the impregnated silver. Subsequently, improvedselectivity for oxidation of ethylene to ethylene oxide may be obtainedby impregnating the activated silver catalyst with a solution of analkali metal promoter, although this is not required.

In one embodiment, the selected neo-acid is reacted with silver oxide,or a basic silver salt e.g. silver carbonate. Although not essential, ithas been found beneficial to include a solubilizing agent in thereaction mixture, such as alkanols, particularly ethanol or methanol.Conveniently, the reaction mixture will contain about 10 volume percentethanol. Generally, it has been found that complete reaction of thesilver with the acid is not achieved. While the formation of the silversalt at first proceeds rapidly, after about 20% of the acid has reactedthe reaction is inhibited. To facilitate additional salt formation thesilver salt which has been formed is separated from the reaction mixtureby precipation. This may be achieved by adding sufficient extra ethanolto the reaction mixture to precipitate substantially all of the silversalt which has been made. The precipitate may be removed by conventionalmeans such as filtering or centrifuging and the solids washed free ofresidual acid.

Although the direct reaction of a silver compound with the neo acid asdescribed above is preferred, the silver salt could be prepared by othermethods familiar to those in the metal soap art such as the doubledecomposition or fusion methods.

The impregnating solution is prepared by redissolving the washed anddried precipitate in a hydrocarbon solvent, such as toluene,ethylbenzene, xylene, cyclohexane, or cumene. It is desirable that thesolvent be capable of holding a large amount of the silver salt in orderto permit preparation of silver catalysts having 7-15 wt % silver in asingle impregnation. However, the proportions of solvent and the silversalt may be adjusted as may be convenient for preparation of thecatalyst and are not considered critical. Typically a weight ratio ofsilver salt/solvent of 1/1-2/1 may be used, particularly about 2/1.Depending upon the solvent, the silver salt, and the ratio of the two,the temperature of the impregnating solution may be at room temperatureor above to provide the desired results. A solvent should also beselected which can be conveniently removed and recovered for reuse.

Impregnation of the selected support is achieved in a conventionalmanner, that is, immersing the support in the silver salt solutiondescribed above for a period of time sufficient to saturate the pores ofthe support. The saturated support is removed from the solution and anyexcess drained off. After the silver salt has been applied to thesupport, the catalyst is activated by heating the impregnated particlesto a sufficient temperature to remove the solvent and to decompose thesilver salt, at least in part, to elemental silver.

Activation of the silver may be carried out by heating to temperaturesof about 200° to 600° C., preferably 250° to 500° C., in the presence ofair or reduced oxygen atmospheres as desirable to control decompositionof the silver salts. The temperatures should be regulated so that thesilver particles are highly active and suitable for oxidation ofethylene to ethylene oxide so that the catalyst can be used, evenwithout the advantage obtained by post-deposition of an alkali metal(s).Preferably, the temperature will be raised gradually to a maximum ofabout 300° C., and held at the maximum temperature for a period of aboutone hour, until the silver particles have reached the desired size andall organic materials have been removed. In a particularly preferredprocedure, the impregnated support is heated to about 100° C. over 1hour, then to about 250° C. over two hours. Air will be passed over thesilver-laden support during activation at a rate sufficient to assureoxygen is present at the surface of the support. Although air is thepreferred gas, other gases may be used, but the presence of some oxygenis considered desirable. It is usual for the temperature to rise afterdecomposition of the silver salts has begun. This temperature excursionmay be controlled by adjustment of the activation conditions. Activationof the impregnated support distributed as a thin-layer on a moving beltis particularly useful since by assuring uniform activation of thecatalyst better average performance can be obtained.

When used the amount of alkali metal on the finished catalyst isgenerally similar to those employed heretofore. Thus, the amountdeposited will be generally up to about 8×10⁻³ gew/kg catalyst,preferably up to about 7×10⁻³ gew/kg, and particularly about 1-6×10⁻³gew/kg (gew=gram equivalent weight). The alkali metals of the periodictable include sodium, lithium, potassium, rubidium, and cesium. Forpurposes of the present invention, the latter three alkali metals areparticularly preferred, especially cesium, although sodium and lithiumare not necessarily excluded. The alkali metal(s) will be supplied asmetal compound(s) which maybe associated with various anions, as forexample hydroxide, nitrates, halides, formates, and acetates,particularly acetates. Conveniently, the alkali metal compounds aredissolved in water or alcohol-water solutions, and preferably ethanolwater solutions containing only enough water to solubilize the alkalimetal compound.

Catalysts prepared by the procedures described above have improvedperformance for use in the production of ethylene oxide by the vaporphase oxidation of ethylene with molecular oxygen. Oxidation reactionconditions such as those previously known in the art may be employed.These usually involve reaction temperatures of about 150°-400° C.,usually 200°-300° C., and reaction pressures in the range of from 0.5-35bar. Reactant feed mixtures usually contain 0.5-20% ethylene and 3-15%oxygen, with the balance comprising comparatively inert materialsincluding such substances as nitrogen, carbon dioxide, methane, ethane,argon, and the like. Only a portion of the ethylene usually is reactedper pass over the catalyst and after separation of the desired ethyleneoxide product and the removal of appropriate purge streams and carbondioxide to prevent uncontrolled buildup of inerts and/or by-products,unreacted materials are returned to the oxidation reactor.

The following examples will illustrate the preparation and use ofcatalysts according to the invention and will support those aspects ofthe method previously disclosed as critical to achieving the desiredresults. Unless otherwise indicated, all parts and percentages are givenby weight for liquids and solids, while for gases compositions are givenas mol %, and flow rates are given in normal cubic meters per hour, i.e.at 0° C. and 760 mm Hg. The fraction of the ethylene converted toethylene oxide is given as the percent selectivity, as is usual in theart.

EXAMPLE 1

A silver impregnating solution is prepared by dissolving 173 gm ofsilver oxide in a solution of 173 gm of absolute ethanol and 520 gm ofneodecanoic acid obtained from Exxon Chemical Co., which contains 67% 2ethyl 2 methyl heptanoic acid. 31% 2, 2 dimethyl octanoic acid, and 2%2, 2 diethyl hexanoic acid. The neodecanoic acid and ethanol are mixedand heated to 80° C. The silver oxide is added in increments withvigorous stirring. Ten (10) drops of 30% hydrogen peroxide is added toclear the solution of prematurely reduced silver. Then 3000 gm ofabsolute ethanol is added to precipitate silver neodecanoate, which isfiltered and then twice reslurried in 3000 gm of ethanol filtered anddried in air. About 85% of the silver oxide is recovered as silverneodecanoate. Two hundred seventeen (217) gm of silver neodecanoatepowder are dissolved in 117 gm of toluene and the mixture is heated to80° C. for impregnation of the support.

One hundred fifty (150) gm of the support material (Norton 5552 as1/4"×1/4" rings) is preheated to 85° C. and immersed in the silverneodecanoate-toluene solution for 20 minutes. The saturated support isdrained and subjected to a programmed heat treatment in air to decomposeorganic residue and deposit silver metal in a form suitable for thefinished catalyst. The impregnated support is exposed for 11/2 hours at100° C. to evaporate toluene, then exposed to a temperature of 250° C.When a temperature of about 200°-225° C. is reached, decomposition ofthe organic material begins, causing the temperature to rise to335°-350° C. After 30 minutes, the catalyst is removed from the sourceof heat and allowed to cool to room temperature.

In order to improve the performance of the freshly-activated silvercatalyst it is impregnated with a solution of cesium acetate in awater-ethanol mixture. This solution is prepared by dissolving 4.19 gmof cesium acetate in 15.8 gm of distilled water. The resulting solutionis mixed with 380 gm of anhydrous ethanol. The resulting solution hasabout 7000 ppm wt of cesium in solution. The described impregnatingsolution is circulated through a bed of the activated silver catalystfor 2 hours. The excess solution is drained and the catalyst issubsequently washed three times with pure anhydrous ethanol. Thecatalyst was dried and then ground to 12-18 mesh particles for testing.The catalyst contains 9% Ag, and 183 ppm Cs (1.39×10⁻³ gew/kg) byanalysis. An additional 100 ppm (wt.) cesium was deposited by saturatingthe dry catalyst with a solution made from 0.005 gm of cesium acetate in0.5 gm water and 9.8 gm ethanol.

A charge of 36 gm of this catalyst is placed in a reactor consisting ofan u-shaped stainless steel tube 5.33 mm internal diameter heated in amolten salt bath. A feed mixture of 0.2% ethane, 14% ethylene, 6.7%oxygen, 5.5% carbon dioxide and 0.5 ppm ethylene dichloride and balancenitrogen was fed over the catalyst at a GHSV of 6000 hr⁻¹. The pressurewas maintained at 20.4 bar and temperature was maintained between230°-280° C.

The results are shown in the following table.

                  TABLE 1                                                         ______________________________________                                                        Cs       Reactor  % EO                                        Catalyst                                                                             Ag wt %  wt. ppm  Temp. °C.                                                                       Outlet                                                                              % Sel.                                ______________________________________                                        1      9        260      236      1.52  82.3                                  ______________________________________                                    

EXAMPLE 2

If free neo-acid is present in the silver salt solution inferiorcatalysts have been produced, as will be seen from the followingexample.

A silver impregnating solution is prepared by reacting 80 gm of silveroxide with a solution of 240 gm of prime grade neodecanoic acid fromExxon Chemical Co. and 80 gm of absolute denatured ethanol from AshlandChemical Co. The silver oxide was added over a period of 25 minutes.Thirty drops of 30% hydrogen peroxide was added to clear the solution.The solution was evaporated until a concentration of 23.2 wt % silverwas reached. For each mol of silver neodecanoate formed, 3 mols of freeneodecanoic acid remained. Instead of precipitating the silverneodecanoate as in Example 1, the solution was used directly to preparea silver catalyst.

Three hundred (300) grams of Nortion 5552 catalyst support was immersedin the reduced solution just described. After withdrawing theimpregnated support, the excess solution was drained off. Then, thesupport was heated in an oven in air at 130° C. for 2 hours, 200° C. for2 hours, 260° C. for 2 hours, and 300° C. for 2 hours.

After cooling, the activated silver catalyst was post-impregnated with acesium alcohol solution and tested as described in Example 1. Theresults are shown in the following table.

                  TABLE 2                                                         ______________________________________                                                                           % EO  %                                    Catalyst                                                                             Ag. wt %  Cs ppm   Temp, °C.                                                                       Outlet                                                                              Sel.                                 ______________________________________                                        2      8.6       154      250      1.52  79.6                                 ______________________________________                                    

The results of the catalyst of Example 2 are inferior to that obtainedin Example 1 where free neodecanoic acid had been removed.

EXAMPLE 3

While the solvent would seem to be merely a vehicle for applying thesilver neo-acid salt to the support, it has been found that the solventalso affects the performance of the finished catalyst, for reasons notunderstood at this time, but which may involve the decomposition of thesilver salts. This example shows the effect of increasing molecularweight on alkyl derivatives of benzene used as solvents for silverneodecanoate.

As in Example 1, silver neodecanoate is prepared as a solid and then 316gms are dissolved in 166 gms of toluene. The resulting solution is usedto impregnate 464 gms of Norton 5552 support. After draining excesssolution, the impregnated support is activated by drying and heating to250° C. in a forced air oven as in Example 1. The activated catalyst ispost-impregnated with a water-ethanol solution of cesium acetatefollowing the procedure of Example 1.

For comparison, a catalyst is prepared using xylene as a solvent. Onehundred sixty five (165) gms of silver neodecanoate is dissolved in 110gms of xylene and 194 gms of Norton 5552 support is impregnated asbefore. The impregnated support is activated by placing it as a singlelayer on a moving metal-mesh belt and exposing it to 500° C. air passingthrough the belt for about 2 minutes. After cooling, the activatedcatalyst is postimpregnated with a water ethanol solution of cesiumacetate as before.

A third sample was made by dissolving 142 gms of silver neodecanoate in95 gms of cumene and impregnating 220 gms of Norton 5552 support. Theimpregnated support is activated by exposing it to air at 500° C. for 2minutes while lying on a moving belt. The activated catalyst waspost-impregnated with a water-ethanol solution of cesium acetate aspreviously described.

Each of the finished catalysts were tested following the procedures ofExample 1 with the following results:

                  TABLE 3                                                         ______________________________________                                        Cat-          Ag.     Cs,    Reactor % EO  %                                  alyst                                                                              Solvent  wt %    wt. ppm                                                                              Temp, °C.                                                                      Outlet                                                                              Sel.                               ______________________________________                                        3 a  toluene  10.3    356    248     1.56  81.5                               3 b  xylene    7.47   212    246     1.49  82.8                               3 c  cumene   8.2     259    236     1.51  83.5                               ______________________________________                                    

Alkali and alkaline earth metals are well known as promoters for silvercatalysts. Since prior art methods of catalyst preparation usuallyemployed aqueous solutions it was possible to use commonly availablealkali and alkaline earth metal compounds which are water soluble. Suchcompounds are not soluble in the organic solvents employed in preparingthe catalyst of the invention. They may be used if they are converted tothe alkali and alkaline earth metal salts of the neo-acid used todeposit silver, or some closely related analog, as will be seen from thefollowing example.

EXAMPLE 4

The cesium salt of neodecanoic acid can be formed by adding 11.27 gm ofan aqueous solution containing 50% cesium hydroxide to 6.50 gm ofneodecanoic acid. A homogeneous solution results, which is evaporated torecover cesium neodecanoate crystals. These crystals are added to anorganic solution containing silver neodecanoate and used to impregnate asupport. Following the procedures of Example 1,165 gm of silverneodecanoate is dissolved in 86.3 gm of toluene. The solution isfiltered to remove undissolved solids and then 0.365 gm of cesiumneodecanoate is added. A clear homogeneous solution results which isstable. The support is impregnated and then activated according toExample 1 to form a promoted silver catalyst which when analyzed isfound to contain 250 ppm wt. cesium.

Although acid-base reactions to form silver salts of neo-acids areconvenient, it is also possible to use other techniques, such as thedouble displacement method illustrated in the following example.

EXAMPLE 5

Sodium neodecanoate is formed by reacting 31.1 gm of neodecanoic acidwith 7.2 gm of sodium hydroxide. The solution is diluted to 500 ml. withwater. An aqueous solution of silver nitrate is prepared by dissolving30.6 gm of silver nitrate in distilled water the diluting with water toa total volume of 500 ml. The two aqueous solutions are mixed, resultingin the formation of silver neodecanoate, which precipitates from thesolution. The solids are filtered, washed with distilled water, anddried. Thereafter, they are used to prepare silver catalysts followingthe methods of the previous examples.

It has been found that practical difficulties limit the application ofthe process of the invention, which should not be considered merely anextension of prior art techniques to higher molecular weight carboxylicacids, as will be seen in the following examples.

EXAMPLE 6

The silver salt of neo-heptanoic acid is formed by reacting 3.87 gm ofsilver oxide with 50 gm of neoheptanoic acid obtained from ExxonChemical Co. in 15 gm of absolute ethanol in a hot water bath maintainedat 80°-85° C. After two hours some of the silver salt has precipitated,but to obtain dissolved salt 1000 ml. of additional ethanol is used todilute the reaction mixture. Since the dissolved salt is insoluble inethanol it precipates and is recovered by filtering and then washed withethanol before use.

The silver neo-heptanoate is less soluble in hydrocarbon solvents thansilver neo-decanoate. A saturated solution is prepared by dissolving 11gm of silver heptanoate in 87 gm of toluene at 85° C. The solution isfiltered to separate any undissolved solids and then used to prepare asilver catalyst. The maximum concentration of silver is only 1.5 wt %,making it difficult to obtain a catalyst having a typical silver contentof 8-15 wt %.

EXAMPLE 7

The silver salt of neopentanoic acid is prepared following the procedureof Example 6. As before, the silver salt precipitated in the reactionmixture. The silver neopentanoate was found to be nearly insoluble inaromatic solvents, ethanol, acetone, and water. Consequently, silvercatalysts could not be prepared according to the method of theinvention.

The preparation was repeated using 2 ethyl hexanoic acid and decanoicacid and results similar to those obtained with silver neopentanoate arefound.

The results obtained with the various acids above are summarized in thefollowing table.

                  TABLE 4                                                         ______________________________________                                                                 Maximum Solubility                                                            in Toluene gm                                        Acid        Structure    salt/gm toluene                                      ______________________________________                                        Neodecanoic                                                                                ##STR1##        2:1                                              Neoheptanoic                                                                               ##STR2##     0.011:1                                             Neopentanoic                                                                               ##STR3##    <0.001:1                                             2 Ethylhexanoic                                                                            ##STR4##    <0.001:1                                             Decanoic                                                                                   ##STR5##    <0.001:1                                             ______________________________________                                    

From the above summary it can be seen that neodecanoic acid isparticularly useful because the silver salt has substantial solubilityin toluene, or similar aromatic solvents, making possible preparation ofsilver catalysts containing 8-15% silver. The other acids produce silversalts which are for practical purposes not useable. Consequently, it isconsidered that the silver salts of neo-acids will be useful only aboveabout C₇ neo-acids. Silver neodecanoate is particularly preferred sincethe acid is readily availably commercially. However, other neo-acidswhich produce silver salts soluble in hydrocarbon solvents may be usedand are considered within the scope of the invention.

EXAMPLE 8

Silver neodecanoate is especially useful since it is quite soluble inaromatic hydrocarbons and the neo-acid is commercially available.However, the silver salts of higher molecular weight neo-acids can beprepared and also appear to be soluble in aromatic hydrocarbons so thatsilver catalysts can be prepared.

The silver salt of neotridecanoic acid is formed by reacting 114.19 gmsof silver oxide with 393 gms of neotridecanoic acid (obtained from theExxon Chemical Co.) in 30 gms of absolute ethanol. As with neodecanoicacid, the neotridecanoic is not a pure compound and is believed tocontain various acids having the "neo" configuration, as previouslydefined, and averaging about 13 carbon atoms. After refluxing for onehour, five drops of 30% hydrogen peroxide is added and a clear solutionresults. The mixture of silver neotridecanoate and unreacted neo-acid ismixed with an excess of ethanol (about 3000 cc) and the silver saltprecipitates, making it possible to easily separate the silver salt fromthe unreacted acid. In contrast to silver neodecanoate which isdistinctively crystalline and forms a suspension in ethanol, silverneotridecanoate forms a compressed tacky mass. The solid is washed withethanol to remove most of the unreacted neo-acid.

A catalyst is prepared by dissolving 194.3 gms of silver neotridecanoatein 77.7 gms of cumene. The solution contains at least 20% more silverthan is possible with silver neodecanoate. Two hundred seventy eight(278) gms of Norton 5552 support is impregnated with the silver solutionin the usual manner. The impregnated support is activated on thepreviously described belt drier.

I claim:
 1. A process for preparing a supported silver catalyst suitablefor the oxidation of ethylene to ethylene oxide comprising:(a)impregnating a support with a hydrocarbon solution of a silver salt of aneo-acid having 7 or more carbon atoms, said solution beingsubstantially free of water and said neo-acid, said support comprisingalumina, silica, silica-alumina or combinations thereof; (b) separatingthe impregnated support of (a) from said solution and heating saidseparated support in the presence of molecular oxygen for a period oftime sufficient to produce an active silver catalyst containing up toabout 15 wt. percent silver.
 2. The process of claim 1 wherein saidsilver salt is silver neodecanoate.
 3. The process of claim 1 whereinsaid silver salt is dissolved in a hydrocarbon solvent selected from atleast one member of the group consisting of toluene, xylene,ethylbenzene, cyclohexane, and cumene.
 4. The process of claim 1 whereinsaid silver salt solution contains no more than about 0.1% water.
 5. Theprocess of claim 1 wherein said support has a surface area of up toabout 2 m² /gm.
 6. The process of claim 1 wherein said silver salt isprepared by reacting a silver compound with said neo-acid in thepresence of a solubilizing agent.
 7. The process of claim 6 wherein saidsolubilizing agent is ethanol.
 8. The process of claim 6 wherein saidsilver salt is separated by introducing a precipitating agent toseparate said salt.
 9. The process of claim 8 wherein said separatedsilver salt is washed to remove free acid.
 10. The process of claim 1wherein said silver salt is prepared by reacting said neo-acid with analkali metal hydroxide to form the alkali metal salt of said neo-acidand thereafter reacting said alkali metal salt with a silver compound toform the silver salt of said neo-acid.
 11. The process of claim 1further comprising the steps of:(c) post-impregnating the activatedcatalyst of (b) with a solution of a compound of at least one alkalimetal selected from the group consisting of Cs, K, and Rb and producinga finished catalyst containing up to about 8×10⁻³ gew of said alkalimetal for each kilogram of said finished catalyst.
 12. The process ofclaim 1 further comprising the step of adding an alkali metal salt of aneo-acid to the impregnating solution of (a), said alkali metal being atleast one selected from the group consisting of Cs, K, and Rb, insufficient amount to produce a finished catalyst containing up to about8×10⁻³ gew of alkali metal for each kilogram of said finished catalyst.13. The process of claims 11 or 12 wherein said alkali metal is cesium.14. The process of claims 11 or 12 wherein said alkali metal content isabout 1-6×10⁻³ gew/kg of said finished catalyst.